By: Thomas Lee » The Internals of Python’s IMPORT_NAME Bytecode

Thomas Lee » The Internals of Python’s IMPORT_NAME Bytecode — Mon, 14 Apr 2008 10:18:12 +0000

[...] was originally planned as a response to this post by Paul Bonser, but grew a little unwieldy (and his comment submission form seems to be [...]

By: Thomas Lee

Thomas Lee — Mon, 14 Apr 2008 09:50:57 +0000

The documentation for import solves this mystery (well, sort of!):

http://docs.python.org/lib/built-in-funcs.html

“Note that even though locals() and ['eggs'] are passed in as arguments, the import() function does not set the local variable named eggs; this is done by subsequent code that is generated for the import statement. (In fact, the standard implementation does not use its locals argument at all, and uses its globals only to determine the package context of the import statement.)”

Essentially, when the IMPORT_NAME is executed for ‘from foo import bar, baz’, the import builtin is called with the fromlist (which is converted to a tuple of strings). to provide custom import handling for your Python programs. For example, you may want to prevent users of your program from writing scripts that import certain modules.

The code in Python/ceval.c for IMPORT_NAME seems to back this up (I’ve annotated it with a few comments …):

    case IMPORT_NAME:
        w = GETITEM(names, oparg);
        /* 1. LOCATE THE __import__ BUILTIN */
        x = PyDict_GetItemString(f-&gt;f_builtins, "__import__");
        if (x == NULL) {
            PyErr_SetString(PyExc_ImportError,
                    "__import__ not found");
            break;
        }
        Py_INCREF(x);
        v = POP();
        u = TOP();
        /* 2. BUILD THE LIST OF ARGUMENTS FOR __import__ USING THE fromlist, globals() AND locals() */
        if (PyInt_AsLong(u) != -1 || PyErr_Occurred())
            w = PyTuple_Pack(5,
                    w,
                    f-&gt;f_globals,
                    f-&gt;f_locals == NULL ?
                      Py_None : f-&gt;f_locals,
                    v,
                    u);
        else
            w = PyTuple_Pack(4,
                    w,
                    f-&gt;f_globals,
                    f-&gt;f_locals == NULL ?
                      Py_None : f-&gt;f_locals,
                    v);
        Py_DECREF(v);
        Py_DECREF(u);
        if (w == NULL) {
            u = POP();
            Py_DECREF(x);
            x = NULL;
            break;
        }
        READ_TIMESTAMP(intr0);
        v = x;
        /* 3. CALL __import__ */
        x = PyEval_CallObject(v, w);
        Py_DECREF(v);
        READ_TIMESTAMP(intr1);
        Py_DECREF(w);
        SET_TOP(x);
        if (x != NULL) continue;
        break;

(hope the code looks okay – you need a “preview” feature )

Comments on: Python IMPORT_NAME bytecode mystery

By: Thomas Lee » The Internals of Python’s IMPORT_NAME Bytecode

By: Thomas Lee